Concern with stress corrosion cracking in high-strength steels and steel weldments has prompted the development of laboratory tests for hydrogen embrittlement of these steels and weldments. Early tests impose a static tensile load on a notched tensile coupon since hydrogen embrittlement failure occurs under a static tensile load for an extended period of time as hydrogen migrates to areas of stress concentration and causes brittle fracture. A chemical bath can be used to induce absorption of hydrogen into the material. The time required for failure of the steel or weldment depends on the absorbed hydrogen concentration, the magnitude of the stress concentration, and the magnitude of the applied stress.
One early apparatus employs a stress-rupture machine which loads a notched tensile coupon statically in tension at 75 percent of its ultimate strength until failure occurs or 200 hours have elapsed. Clear disadvantages of this type of tensile testing apparatus include the high cost of a stress rupture machine and the need to prepare test coupons with threads or shoulders for gripping on the machine for tensile loading.
To overcome these problems, researchers have developed a method whereby a notched coupon is loaded in tension by bending. Because the coupon is not loaded axially in tension, no threads or shoulders for gripping are necessary. The coupon is simply slipped into the testing device for loading. In addition, because the load requirement is lower, the cost of such a testing device is usually a fraction of the cost of a stress rupture machine.
In the article, L. Raymond & W. R. Crumly, "Accelerated, Low-Cost Test Method for Measuring the Susceptibility of Steels to Hydrogen Embrittlement," American Society for Metals, 1982, is shown a loading frame that loads a notched sample in tension by bending it with a pair of loading arms. The sample is notched at the mid-section and clamped at both ends by the pair of loading arms. The loading arms are pivoted at the bottom and rotate as loading is applied by turning the instrumented bolt at the top end of the loading arms, bringing them together. This effects a bending moment on the clamped sample, causing the notch to open in tension. A rising step-load procedure is used to load the sample with increasing stress-to-failure so that the susceptibility to hydrogen assisted cracking can be indexed with respect to the failure load.
Although this type of loading frame has numerous advantages over the tensile testing apparatus, it also has certain disadvantages. The load transfer between the two loading arms and the sample is poor due to the inherent problems of the loading frame linkage. Each of the two loading arms is pivoted at the bottom. For each arm, there are top and bottom sections connected by screws. The bottom section is pivoted at the lower end and has a slot for holding the sample horizontally at the upper end. To place the sample in position for loading, the two top sections of the loading arms are first removed. After the sample is placed horizontally in the slots provided by the bottom sections of the two loading arms, the top sections are placed on top to hold the sample in place, using a set of screws that penetrate through the bottom sections into the top sections; in other words, the sample is clamped between the top and bottom sections. As loading is applied by turning the instrumented bolt and a bending moment is transferred to the sample, a large amount of mechanical slop can result in the linkage due to the many separate components in the overall loading frame linkage through which the load is transferred. This can be caused by bolt misalignment, and friction and slippage at various contact surfaces within the linkage of the loading frame, leading to false or inaccurate load readings.
It is intended that the apparatus apply four-point bending to the sample. However, the connections between the pair of loading arms and the sample are neither perfectly fixed clamps nor simple supports at four points. This causes slippage between the sample and the loading arms, resulting in a loading mode that is not truly a four-point bending. In addition, this slippage leads to inefficient load transfer to the sample. Consequently, false load readings and possible changes in sample displacement may result.
The placement of instrumentation for load measurement further contributes to the inaccuracy in load readings. The load measurement is taken at the instrumented bolt, which is far removed from the sample. Inaccuracy in load readings results from the large separation between the bolt and the sample and the inefficient load transfer due to mechanical slop and slippage between the point of measurement and location of the sample.
In this type of apparatus, sensitive crack detection in the test sample requires accurate load measurement. Further, when the sample is tested under constant displacement, any movement of the sample will compromise the integrity of the measured data. Movement can result from sample slippage, temperature fluctuations, and bending or relaxation in the test apparatus. As described above, these problems are present in this loading structure, creating problems of inaccuracies in load and displacement measurements.
Furthermore, the process of installing the sample into the loading frame is rather complicated and cumbersome because one must loosen the screws and remove the top sections of the two loading arms and the instrumented bolt structure, which constitute the bulk of the weight of the apparatus. After the sample is placed in the slots of the two bottom sections, the top sections of the loading arms and the bolt structure are turned over to be clamped onto the sample. Because the loading frame must be rigid and substantial in weight, it is generally very heavy, especially for larger samples, making it difficult or impractical to deposit and remove samples. This also restricts the size of samples that can practically be tested with this loading frame. The fact that much of the positioning and adjustment required in depositing and removing the sample is performed inside the chemical bath further complicates and slows down the procedure.